Highly active anti-retroviral therapy (HAART) has been remarkably effective for managing HIV infection / AIDS. However, patient compliance with HAART is often variable due to treatment- related complications. This is a serious problem that facilitates the emergence of drug resistance. One issue that contributes to HAART-associated complications is the poor metabolic stability and low cellular penetration of the HIV protease inhibitors. Our group has been exploring a new method for addressing these limitations. This strategy is directly inspired by the pharmacological properties of the natural product, FK506, which has a surprisingly long halftime in humans (t1/2 ~ 40 hrs) despite being an excellent substrate for P450 enzymes in vitro. We recently wondered whether this apparent contradiction might arise from this compound's high affinity for the FK506-binding protein (FKBP). Blood cells, including both erythrocytes and leukocytes, express unusually high levels of FKBP but they are virtually devoid of P450 enzymes. Therefore, we hypothesized that affinity-driven accumulation within this protected cellular niche might limit exposure to key metabolic enzymes and, thereby, extend drug lifetime. Moreover, FK506 is rapidly absorbed (~1 hr), highly penetrant to biological membranes and naturally targeted to leukocytes via its FKBP-binding moiety. Together, these properties appear to align with the major problems facing HIV protease inhibitors. Based on these observations, we tethered an FKBP-binding group to an amprenavir analog, creating a bifunctional molecule that can bind both FKBP and HIV protease. We found that the resulting compound retained anti-protease activity (IC50 ~ 20 nM). Moreover, it was now sequestered into blood cells (by at least 8-fold) and its half-life was increased by ~ 20-fold in mice. The lifetime of this compound was superior to that of ritonavir-boosted amprenavir and, moreover, its metabolic stability was now independent of ritonavir co-administration. Based on these promising initial findings, we now propose to carefully explore the molecular mechanisms governing this behavior. Specifically, we reason that cellular partitioning and lifetime are dictated, in part, by the affinity of the compound for FKBP. To explore this central hypothesis, we propose the following specific aims: (1) synthesize a collection of FKBP-binding amprenavir derivatives and measure their relative affinities for albumin, FKBP and HIV protease and (2) explore how cellular partitioning correlates with binding affinities and relative protein expression levels. From these observations, we expect to understand the key structure-activity relationships in the context of the ternary complex (i.e. FKBP-drug-HIV protease). The immediate goal of this study is to create potent, safe and long-lived HIV protease inhibitors that selectively target FKBP-expressing, HIV-infected cells. This study is significant because it addresses an important problem in the treatment of AIDS. The proposed work is innovative because it will explore a fundamentally new, natural product-inspired strategy.